[web] First pass at styling ConfirmLeaveThreadModal

Summary:
Started with styles borrowed from thread-settings-modal.css and made some tweaks so it looks decent.

Before:

After:

Test Plan:

1. Created a "leavable" thread in my dev environment
2. Clicked "Leave thread" in menu
3. Ensured that the ConfirmLeaveThreadModal looked as expected
4. Clicked the "Leave thread" button and confirmed that I had left the thread.

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3705